9.4. Generic driver (sg)

All types of SCSI devices are accessible via the sg driver. This means devices such as CDROM drives can be accessed both via the sr and sg drivers. Other SCSI devices such as scanners can only be accessed via the sg driver. The sg driver is capable of recognizing 256 SCSI devices. Any number of devices (up to the overall limit of 256) can be added after the sg driver is loaded.

See reference W4 for the SCSI Generic (sg) driver documentation (also found there is the sg_utils package). For SCSI standards see reference W1 and for a book on the subject of SCSI programming and pass through mechanisms see reference B3.

The sg driver in lk 2.4 is "version 3" which adds an additional interface structure and some new ioctl()s. The most interesting new ioctl() is SG_IO which sends a SCSI command and waits for its response. See the Linux Documentation Project site: www.tldp.org/HOWTO/SCSI-Generic-HOWTO/ for a full description of the sg driver. A (possibly later) version of this document can be found at www.torque.net/sg/p/sg_v3_ho.html.

The abbreviation "sg" is used within the kernel to refer both to the SCSI generic driver and the scatter-gather capability offered by many modern IO devices (usually associated with DMA). The context usually makes it clear which one is being referred to. As an example, note the contorted sg ioctl() named SG_GET_SG_TABLESIZE where the second "SG" refers to scatter gather.

The public interface for sg is found in the file:
/usr/src/linux/include/scsi/sg.h. Depending on the distribution this may or may not contain the same information as
/usr/include/scsi/sg.h which is controlled by the GNU library maintainers. If these 2 files are not the same use the former header file. Those writing applications based on sg should see its documentation for more on this matter.

The sg driver registers all SCSI devices (with a current maximum of 256) as they are seen. Each newly registered SCSI device gets allocated the next available minor device number. At least initially this will be the same sequence that devices are displayed in mid level's
cat /proc/scsi/scsi. The sg devices device mapping can be seen with cat /proc/scsi/sg/devices or
cat /proc/scsi/sg/device_strs. Differences between cat /proc/scsi/scsi and sg orderings will appear when a low level driver is removed (e.g. rmmod aha1542) or when a device is removed with remove-single-device. The sg driver will leave remaining SCSI device mapping to minor device numbers unchanged. This potentially leaves a "hole" in the sg mapping. An example follows:

Notice how the sg driver maintains the row positions of the remaining devices in the "device_strs" output. So when the Pioneer dvd player is removed, a hole opens up in the sg device mapping which is not reflected in the cat /proc/scsi/scsi output. That "hole" corresponds to the device name
/dev/sg1.

The new sg_io_hdr interface includes a data transfer residual count field called "resid". Only some lower level adapters support this feature and those that don't always yield zero in this field. At the time of writing the advansys, aha152x and the sym53c8xx drivers support this feature.

9.4.1. sg boot parameters

The sg driver maintains a reserved buffer for each open file descriptor. The purpose is to guarantee applications that data transfers up to the size of the reserved buffer will not fail for lack of kernel memory. This is important for applications like cdrecord that cannot easily recover (the CDR) from a ENOMEM error.

In the absence of the boot parameter 'sg_def_reserved_size' or the sg module parameter 'def_reserved_size', then each time a sg file descriptor is opened the reserved buffer size is inherited from SG_DEF_RESERVED_SIZE which is defined in include/linux/sg.h.

The SG_DEF_RESERVED_SIZE define value can be overridden by this kernel boot option:

sg_def_reserved_size=<n>

9.4.2. sg module parameters

When the sg module is loaded the SG_DEF_RESERVED_SIZE define value can be overridden by supplying this option:

def_reserved_size=<n>

9.4.3. sg proc interface

All the following files are readable by all and produce ASCII output when read. The file 'def_reserved_size' is also writable by root. The ASCII output has been formatted in such a way as to be human and machine readable (and hence a compromise). Use Unix commands of the form cat device_hdrs devices to see the output of tables.

All the above files are owned by root and readable by all while def_reserved_size is writable by root. For the devices and device_strs files the first row output corresponds to /dev/sg0 (sg minor device number 0). The second row output corresponds to /dev/sg1, etc. For the hosts and host_strs files the first row output corresponds to host (adapter number) 0, etc. For numeric tables a missing device or host is indicated by a row of "-1" values. For string tables a missing device or host is indicated by a row containing "<no active device/host>".